There are many applications today that utilize displays formed from an array of individually addressable pixels. For convenience of description these are hereafter referred to as “pixeled displays.” The optical images to be presented by such displays are formed by activating various combinations of the individual pixels. An electrical or other signal causes portions of the various pixels to switch ON or OFF. The switched portions of the pixel may become transparent or opaque to a backlight or become luminous or dark, depending upon the type of pixel employed. In many display types, intermediate levels between full ON and OFF are also possible, for example by analog switching or pulse width modulation. In this way a light image is presented to the viewer. Liquid crystal (LC) and plasma flat panel displays are non-limiting examples of such pixeled displays. The present invention is also applicable to other types of pixeled displays and such are intended to be included. The prior art and the present invention are described herein for the case of a transmissive type liquid crystal display (LCD) but this is merely for convenience of explanation and not intended to be limiting.
FIG. 1A illustrates typical individual pixel 10 of a transmissive type liquid crystal display (LCD) according to the prior art and FIG. 1C illustrates liquid crystal display (LCD) panel 20 formed from an array of prior art pixels 10 of FIG. 1A. While only a small number of pixels are shown in FIG. 1C, it may be thought of as representing the whole LCD display or any sub-portion thereof. The exact number of pixels is not critical for the present invention. Individual pixels 10 comprise (e.g., in a transmissive LCD panel) region 12 that may be made transmitting or luminous by, for example, electrical activation of the pixel, and surrounding region 14 that is ordinarily opaque and dark and usually contains the various electrical leads and other circuitry needed to drive the pixel as well as light blocking layers covering various portions of the display. These light blocking layers are often used to mask edge effects in the pixels or to shield the circuitry from incident light. Thus, depending upon whether pixel 10 is activated or not, region 12 may be transparent (luminous) or opaque (dark). By being switchable between ON and OFF, region 12 is considered the active aperture or switchable region of the pixel. The terms “active aperture” and “switchable region” are used interchangeably herein to refer to that portion of the pixel whose luminosity or transparency may be altered by an electrical signal. Region 14 is generally opaque and dark and is therefore the inactive aperture or non-switchable region of the pixel. In some displays, region 12 is ordinarily opaque (dark) and becomes transparent (luminous) upon activation and in some it is ordinarily transparent (luminous) and becomes opaque (dark) on activation. For the present invention, it does not matter which arrangement is used. For convenience of explanation, it is assumed hereafter that region 12 (and its equivalents in the present invention) is ordinarily opaque (dark) when in the OFF state and becomes transparent (luminous) when activated, that is, when switched into the ON state, but this is not intended to be limiting. FIG. 1B represents another typical prior art aperture configuration for pixel 10, differing from FIG. 1A only in that the active aperture is reduced in the vicinity of portion 16. Portion 16 represents typical loss of active pixel aperture in an active matrix display, and is usually occupied by a small electronic driver circuit or region (e.g., one or more thin film transistors referred to as TFTs) that activates pixel 10 and by any associated light blocking structures. The presence or absence of portion 16, and the degree to which it impacts the corner of the active aperture varies with the details of prior art displays. In general, it is desirable to minimize the size of portion 16, thereby maximizing the active aperture. The aperture ratio (AR) of a pixel is defined as the proportion or percentage of the total pixel area that is switchable and can be made transparent (luminous). In the case of pixel 10, the AR is the area of region 12 divided by the sum of the areas of regions 12 and 14, or in other words the AR is the ratio of the active pixel aperture (switchable region) to the total pixel aperture, where total pixel aperture is the sum of the active aperture (switchable region) plus inactive aperture (non-switchable region). The aperture ratio (AR) is an important property of the pixel (and therefore the whole display) since, other things being equal, the AR determines the brightness of the display for a given drive level. For the present discussion, the active aperture is considered to be transmitting or transparent even if it is not 100 percent transmissive. Many factors impact the transmittance of the active aperture. In an LCD, for example, the transmittance of the active aperture region may be reduced by polarizers, filters, pixel electrodes (either transparent or interdigitated with very fine spacing), spacer balls, alignment layers, microscopic alignment features and other structural components, which are intrinsic to the function of the active aperture. As such, these are considered as affecting the transmittance but not the area of the active aperture. For example, any films or microscopic opaque structures in regions 12 of FIGS. 1A and 1B are considered to not alter the areas of regions 12 if the films or structures are necessary to sustain the intrinsic operation of the device within the active aperture, regions 12.
Pixeled displays have inherent structure. This can be understood by considering a view of display 20 along a particular direction, as for example along line 22 through row 21 of pixels 10 of FIG. 1C. While line 22 is shown as being parallel to row 21 of display 20, this is not intended to be limiting. Other orientations may also be used. For convenience of explanation the suffix “H” is used to refer to periodicity along the horizontal axis (e.g., the rows) of the display and the suffix “V” is used to refer to periodicity along the vertical axis (e.g., the columns) of the display. Persons of skill in the art will understand based on the explanation herein that the designations “horizontal” or “vertical” are merely convenient labels for a set of orthogonal axes and need not correspond to any particular direction or plane in space. When all pixels 10 of display 20 are ON, i.e., transparent or luminous, then optical response plot 24 of FIG. 1D shows the light/dark structure presented by the display along line 22 as a consequence of its physical structure. Light signals 23 (1=ON) are provided by transparent or luminous regions 12. These are separated by dark signals 25 (0=OFF) provided by opaque or dark regions 14. Optical periodicity 26H′ between adjacent light (or dark) regions in FIG. 1D is the same as physical periodicity 26H of pixels 10 in display 20 in FIG. 1C and is an inherent property of the physical structure of display 20. FIG. 1E illustrates optical output 27 having optical periodicity 28H′ along line 22 when every other pixel is ON (portion 23′) and every other pixel is OFF (portion 25′), corresponding to physical periodicity 28H of FIG. 1C. In FIG. 1E, OFF portion 25′ corresponds to dark regions 14 and an intervening region 12 that is not activated, i.e., that is OFF. Periodicity 28H, 28H′ determines the finest grained, the most detailed, optical output that can be presented by display 20. In the example of FIGS. 1C-E, dimension 28H, 28H′ is twice dimension 26H, 26H′. In display 20 using square pixels 10, horizontal periodicity 26H, 28H and vertical periodicity 26V, 28V are substantially the same, but this is not essential.
The above-described inherent structural periodicity is a significant limitation of pixeled displays since it can create undesirable and distracting visual artifacts in the display output that have nothing to do with the information desired to be presented. Examples of such artifacts are Moiré patterns, “screen door” effects, and the like, which are well known in the art. Prior art attempts to minimize these artifacts have involved using smaller pixels, providing diffuser screens or anti-glare films on the display output, providing very high aperture ratios, and so forth. While these may be helpful in ameliorating some of the inherent artifacts, they have undesirable side effects such as for example, decreased aperture ratio, decreased display efficiency, increased complexity and cost and, sometimes, other visual artifacts (e.g., a speckled appearance). Thus, there continues to be a need for reducing the visual artifacts arising from the inherent structure of pixeled displays.
Accordingly, it is desirable to provide an improved display and method, especially for reducing the undesirable optical artifacts arising from the physical pixel structure of the display. In addition, it is desirable that the improved display and method be simple, rugged and reliable and not require an increase in the number of pixels being addressed. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.